The present invention is in the field of data communications, and more particularly, relates to the synchronization of terminals in ring communications networks.
Ring communications networks generally include a plurality of terminals coupled in series along a unidirectional signal path. In such communications rings, any of the terminals coupled to the ring may transmit or receive data on the ring according to a ring protocol. In the unusual cases where the terminals are all geographically close together, it is a relatively simple matter to synchronize all the terminals, for example, by providing the clock (or timing) signals for use at the respective terminals from a common oscillator. However, in a more typical ring network environment, there is no readily accessible master oscillator. To achieve interterminal synchronization in such systems timing information is added to the transmitted data. In the prior art for such systems, the ring timing information is inserted by a single terminal. All other terminals adapt their transmit rates to the observed incoming data rates. This approach suffers from the disadvantage of having to identify a crucial terminal.
In an alternate prior art approach, all terminals adapt their transmit rates to the observed identified timing source. In effect, the ring topology permits a distributed averaging of the transmit rate. This approach suffers from its sensitivity to the ring delay and from the timing interdependence among the terminals. In particular, an aberrant terminal can produce a difficult to diagnose error situation.
In an alternate prior art approach, a distributed averaging of the transmit rate is achieved where each terminal has an invariant local oscillator whose frequency is some multiple of the transmit bit rate. A terminal's transmit bit rate is then discretely adjusted in accordance with the observed input data rate. The adjustment occurs in time blocks so that the resultant bit rates through the terminal are varied during those blocks while being substantially fixed at other times. As a result, during the blocks, data packets at various terminals might have the same fixed number of bits pass through, but the bits would have adjustable lengths; adjusted so that there is a predetermined nominal clock rate around the ring. In effect, such systems circulate a synchronization word having a fixed number of bits, where the respective bits are discretely variable in length, due to the adjustment. Between synchronization words, the data is clocked at a fixed frequency. While such systems are effective in many applications, there are disadvantages. For example, the sudden introduction of adjustable length bits requires relatively high bandwidth and also prohibits many forms of transceiver design.
It is an object of the present invention to provide an improved bit synchronization system for a ring communications network.